FR2512003A1 - PROCESS FOR PREPARING ALUMINUM OXYNITRIDE AND PRODUCT OBTAINED - Google Patents

Abstract

THE PROCESS FOR PREPARING A PRACTICALLY HOMOGENEOUS ALUMINUM OXYNITRIDE POWDER OF THE INVENTION, INCLUDES THE REACTION OF GAMMA ALUMINUM OXIDE WITH CARBON IN THE PRESENCE OF NITROGEN, AND THE DIVISION OF THE POWDER OBTAINED IN PREDIMENSIONAL PARTICLES . IT ALSO CONCERNS A PROCESS FOR THE PREPARATION OF AN OPTICALLY TRANSPARENT SUSTAINABLE BODY FROM THIS POWDER INCLUDING THE STEPS OF FORMING A GREEN BODY OF THE SAID POWDER AND OF FRITTING THE GREEN BODY UNDER NITROGEN FRUIT AND IN THE PRESENCE OF ADDITIVES WHICH PROMOTE THE POWDER . THE PREFERRED ADDITIVES ARE BORON, IN ELEMENTARY OR COMPOUND FORM, AND AT LEAST ONE ELEMENT CHOSEN FROM YTTRIUM AND LANTHAN OR THEIR COMPOUNDS. SINTERED POLYCRYSTALLINE CUBIC ALUMINUM OXYNITRIDE HAS A DENSITY GREATER THAN 99 OF THE THEORETICAL DENSITY, A DIRECT TRANSMISSION OF AT LEAST 50 IN THE RANGE OF 0.3 TO 5MICROMETERS, AND A RESOLUTION ANGLE OF 1MRAD OR LESS. APPLICATION TO THE MANUFACTURE OF DOMES, WINDOWS OR TRANSPARENT ENVELOPES FOR MISSILES AND STEAM LAMPS.

Description

The present invention relates to ceramic materials

  transparent and sustainable

  compounds in applications requiring high

  transmission and image formation in the visible spectrum and the infrared spectrum These conditions may be required in military and industrial applications For example, the transparent domes at

  infrared sensors are needed for missiles and

  transparent veloppes are needed in different

  types of vapor lamps Many trans-

  parents are not sustainable enough for these

  complications; thus, research has been oriented towards the development of transparent ceramic materials. Although many ceramic materials meet the criterion of durability, they are not sufficiently

  transparent for these applications For example, aluminum

  mine is a very hard matter, but the main

  blemish is that it is not transparent enough and

  diffuse light to an excessive degree Another consideration

  which should be taken into account for a possible subject

  ble is the manufacturing price; thus, processes that require individual treatment of these windows are

  likely to remain in the state of unrealis-

  from the economic point of view

  forging and hot pressing are not advan-

  Thus, there remain the processes of discrete

  tinus as an advantageous alternative feasible and the sintering itself lends itself to the manufacture of various units in a single operation However, the sintering of transparent ceramic materials is not very well known neither

put into practice.

  Aluminum oxynitride is a promising material

  for applications requiring transmission power

  The only prior art Connoe experiment for the production of a sintered aluminum oxynitride body is found in US Pat. No. 4,241,000 in which precursor powders are blended. and the sintering step is used both to react and sinter the precursor powders to produce an aluminum oxynitride body. The problem is that the resulting material is not sufficiently transparent

  for the applications mentioned above.

  These and other problems are solved

  by the present invention which provides a method of

  forming a substantially homogeneous cubic aluminum oxynitride powder, which is particularly useful for sintering the powder to produce a transparent and durable ceramic window. It has been found that starting from a substantially homogeneous powder of oxynitride

  of aluminum, as prepared in this

  vention and using specific additives, a sufficiently transparent window could be obtained

  in the visible spectrum only in the infrared spectrum.

  The present invention provides a method of

  homogenous aluminum oxynitride paration comprising introducing aluminum oxide powder and carbon black into a reaction chamber, introducing nitrogen into the chamber, and heating the chamber to

  react the powder and the gas in order to obtain a

  reacted dre consisting essentially of oxynitride

  The reactive powder can also contain

  up to 15 percent by weight of aluminum oxide and aluminum nitride such that the ratio of aluminum oxide to aluminum nitride is in the composition range of oxynitride aluminum

cubic.

  The present invention further provides a method of

  Transparent body fabrication of oxynitride

  sintered aluminum consisting of preparing a mixture of oxy-

  of aluminum and carbon black, to react the mixture in the presence of nitrogen and at a temperature between 1550 and 18500 C, to form a green body of predetermined shape from the mixture, to place the green body in a sintering chamber, to introduce

  doping additives in the chamber, the additives include

  one or more elements selected from yttrium and lanthanum or compounds thereof, and sintering the green body at a temperature above 19000 C, but below the solidus temperature of aluminum oxynitride. preferably, the doping agents are in the vapor phase during part of the sintering step

  and the vapor is transported to the body and diffuses in that

  The doping additives do not account for more than 0.5 percent of the weight of the green body. The preferred starting mixture has a carbon content of between 5.4 and 7.1.

  preferably, the mixture having a weight of

  git is broken down into particles of size between 0.5 and 5 micrometers, and is heated in air or

  oxygen to eliminate any organic impurities

to be present.

  In addition, the present invention provides a cubic aluminum oxynitride body having a density of

  less than 98% of the theoretical density, a di-

  at least 50% in the wavelength range of 0.3 to 5 micrometers, and an image resolution of 1 mrad

or less.

  The present invention produces a powder practic-n

  homogeneous amount of cubic aluminum oxynitride in

  reacting gamma aluminum oxide with carbon in

  nitrogen atmosphere More particularly, aluminum oxide

  Minium (alumina) and carbon black are dry blended, for example, in a Patterson-Kelly jacketed blender for up to two hours. Preferably, the aluminum oxide has a purity.

  at least 99,98% and an average particle size

  0.06 micrometer, and carbon black has a pure

  at least 97.6% with a content of 2.4% volatiles and an average particle size of 0.027 micron. The carbon content of the mixture may range from 5.4 to 7.1 percent by weight. preferred blend contains 5.6 percent by weight of carbon black and 94.4

  percent by weight of aluminum oxide The mixture of

  of aluminum and carbon is placed in an alumina crucible and is reacted in a circulating nitrogen atmosphere at temperatures of 15500 to 18500 C for up to two hours at maximum temperature. In the first step, a temperature of about 15500 C is used for about one hour, so that, for an appropriate ratio of alumina to carbon, the gamma aluminum oxide is unstable at room temperature. only partially reacts with carbon and nitrogen to form both alpha aluminum oxide and aluminum nitride One hour treatment at 15500 C is sufficient to convert the correct amount of A 1203 to Al In the second step, a temperature of 17500 C or up to the temperature of the aluminum oxynitride solidus (2140 ° C) is used for about 40 minutes, so that the alpha aluminum oxide and the nitride of aluminiu m of combine to form

  cubic aluminum oxynitride.

  The reacted material resulting from the treatment

  is mainly composed of aluminum oxynitride

  cubic minium, but it may also contain alumina

  and / or aluminum nitride in amounts up to 15 percent by weight, such that the ratio of aluminum oxide to aluminum nitride is within

  the composition range of cubic aluminum oxynitride

  The amount of alumina and aluminum nitride can be regulated by the heat treatment and the amount of aluminum nitride produced in the first stage of the process.

  which, in turn, depends on the quantity of car-

  bone contained in the starting mixture.

  For a first step using the treatment

  preferred hour at 15500 C, except for the sample

  5, which is treated at 1620 ° C, Table I illustrates

  the effect of using different quantities of car-

  in the starting mixture and the different temperatures

  eratures during the second stage of heat treatment.

TAB L E A U I

  Sample% by weight Temp Time Ai N A 1203 Carbon ALON (PC) (min)%%%

1 5.6 1750 40 3.2 10.0 86.8

2 7.1 1750 40 4.0 O 96.0

3 6.5 1750 40 1.88 O 98.12

4 5.9 1750 40 0.85 O 99.15

5.6 1820 40 Traces Traces 99.9 +

  The preferred heat treatment gives a composition

  comprising substantially 100% aluminum oxynitride

  and corresponds to Sample 5 Another composition

  The preferred composition obtained is that of Sample 1. The resulting aluminum oxynitride powder consists of

  agglomerated particles which are easily separated

  grinding with logs, especially in

  particles between 0.5 and 5 micrometers

very.

  The reacted material is ground with

  in polyurethane-lined or rubber mills using methanol as the fluid of

  grinding and grinding balls with a high aluminum content

  The grinding time is 16 hours The ground powder passes through a sieve with a mesh size of 0.037 mm and is

  dried at 65 ° C. for a period of up to 24 hours.

  After drying, the powder is heated in air at 600 ° C.

  for 2 hours to remove organic impurities.

  Sintering aids are added at this time in the form of small amounts of doping additives previously selected in amounts reaching

  0.5 percent by weight of the aluminum oxynitride powder

minium.

  The additive may also comprise an element selected from yttrium and lanthanum, or compounds

  of these It is thought that one can use

  Other elements of the lanthanide series are preferably used. The oxide of the selected elements can also be used. A combination of the additives of

  doping provided that the total amount of additives does not exceed

  not exceed 0.50 percent by weight A combination of

  For example, 0.08 percent by weight of yttrium oxide (Y 203) and 0.02 percent by weight of lanthanum oxide (La 203) can alternatively be added during the grinding process. balls of the

aluminum oxynitride powder.

  The aluminum oxynitride powder containing the additives is placed in rubber molds of predetermined shapes and isostatically compressed at pressures greater than 103.5 M Pa to produce green bodies for use in sintering The green bodies manufactured are placed in containers in the room

  sintering vessels are made up entirely of

  Boron nitride, in part boron nitride and partly molybdenum metal. Sintering is then carried out in a stagnant nitrogen atmosphere.

  0-34.5 k Pa To obtain a virtually trans-

  parent, the sintering temperatures are greater than 1900 ° C, but lower than the temperature of solidus

  aluminum oxynitride, which is about 2140 ° C.

  Sintering is carried out for a minimum of 24 hours

and a maximum of 48 hours.

TABLE II

  Bchan% of transmittedEpais Density Psolu-

  tillon temp hd direct dy ation _ 23 1-3 C h 4 micrometers (nam) _% optie

  1 none none 1930 23 cpaque 1,7 98 -

2 0.08 0.02 1930 1 5 0, 82 98 + -

  3 0.08 0.02 1930 24 80 1.45 99 + <1 mrad 4 0, 25 1930 48 53 1.35 99 + <1 imrad 0, 08 0, 02 1730 3 cpaque 1.5

6 0.08 O ó 021910 8 5 0.8 98 -

  Table II shows to some extent the effect of additives, time and temperature on

  the resulting transparency of the aluminum oxynitride.

  The density is measured by the method of Archimedes, the

  transmission is measured using a spectrophotometer

  Perkin-Elmer 457 infrared network photometer and

  the resolution angle is measured using the "Stan-

  dard USAF 1951 Resolution Test Pattern "Temperatures

  are expressed with an accuracy of 10 C

  the temperature of 1900 C is the minimum temperature that has

  proved to be certain to produce a transparent

  having the appropriate content of Y 203 and / or La 203 La

  optimum amount of additive is the minimum quantity

  necessary to produce a liquid phase at the grain boundaries initially, although it is not present as a second phase after sintering Although 0.1 per cent by weight gives the best results, smaller amounts in trace form as low as 0.05 percent by weight seems to be suitable. In other words, a liquid phase is formed at or near 1900 C, which favors rapid densification and elimination.

  pores This liquid phase disappears, Y and The

  in solid solution with aluminum oxynitride.

  This liquid phase sintering process is assumed to be performed at the sintering temperature at the beginning of the sintering process. Next, solid state diffusion is the means by which the remaining porosity is achieved.

  is eliminated and significant transparency is achieved.

  Removal of porosity by solid state diffusion is a much slower means of pore removal, so longer durations are required.

  a duration of 24 hours being the preferred minimum duration.

  This is confirmed by Samples 2 and 6 in

  which, even if a correct amount of addi-

  the samples remain translucent because the duration

  the sintering step is limited to 1 and 8 hours

  tively. It should be noted that the additives described above do not have to be mixed

  with aluminum oxynitride powder before frying

  not in direct contact with the green body.

  Here again, it suffices that the additive chosen is available

  in the sintering chamber for doping oxy-

  In fact, the unexpected improvement in the transparency of the sintered aluminum oxynitride was discovered after the sintering of a green body,

  strictly composed of aluminum oxynitride powders

  nium, and with an adjacent green body containing oxy-

  of yttrium on a boron nitride platform.

  Thus, the present invention is considered to be

  other processes for introducing additives into the sintering chamber to produce doping in situ

  steaming of the compact aluminum oxynitride.

  An explanation of in situ vapor doping

  by the presence of specific additives intended to

  The sintering is assumed to be as follows: at sintering temperatures, the aluminum oxynitride mixture has an extremely high vapor pressure of gaseous Alx OY Alx OY gas reacts with the neighboring boron nitride present in the vessel producing B 203 gaseous and / or Al Bo gaseous plus AlN solid vapors of B 203 and / or Al BO are passed to the aluminum oxynitride and react with it giving a liquid phase at the grain boundaries which improves the early stages of sintering In the case where yttrium oxide is used as an additive, B 203 also reacts with yttrium-doped aluminum oxynitride or pure Y 203 when producing

  YBO 2 gas YBO 2 vapor is transmitted to the oxygen

  aluminum nitride and dope with boron and yttrium.

  In the case of the use of other elements as additives, B 203 reacts in a similar way giving a corresponding doping of the oxynitride

  It is believed that this doping additive favors

  make the final stages of sintering by causing either

  a slowdown in the level of the solute, ie

  second-phase pits to delimit grain boundaries and thus prevent excessive grain growth that could otherwise clog pores

inside the grains.

  Another explanation is that yttrium or

  its compounds causes the formation of a liquid phase.

  This liquid phase promotes a rapid densification and a significant elimination of the pores in the first

  sintering stages so that during the final stages

  of sintering, there is less porosity to be removed

  It is possible to obtain a high density and a high degree of transparency. In this mechanism, boron is only necessary for the transfer of yttrium.

with aluminum oxynitride.

  The method of the present invention avoids many of the normally associated problems

  to the preparation of aluminum oxynitride in

  aluminum oxides and aluminum nitride, such as the various degrees of purity, the large particle size and the broad size distribution range of the commercially available aluminum nitride, the long reaction times necessary to form aluminum oxynitride, and the long durations of

  grinding necessary for the reduction of the

  which, in turn, increase the content of

  inorganic impurities of aluminum oxynitride.

  In addition, the process of the invention reduces manufacturing costs by avoiding the use of the more expensive aluminum nitride as a starting ingredient, by requiring shorter reaction times to form aluminum oxynitide and requiring a

  less grinding time to obtain a homogeneous powder

  sinterable gene, of particle size

  The aluminum oxynitride powder prepa-

  The process of the present invention also improves the reproducibility of the sintering process.

  and improves the transparency of the sintered product.

12003

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Claims (20)

  1 Process for the preparation of aluminum oxynitride   homogeneous medium, characterized in that aluminum oxide powder and carbon black are introduced into a reaction chamber; nitrogen is introduced into said chamber; and heating the chamber to react the powders and gas to produce a reacted powder consisting essentially of aluminum oxynitride. Process according to Claim 1, characterized in that the reacted powder may contain up to 15 percent by weight of aluminum oxide and aluminum nitride in such a way that the ratio of the oxide   aluminum nitride is located in the   composition of cubic aluminum oxynitride.   Process according to Claim 2, characterized in that the aluminum oxide and the carbon black are   introduced in the form of an intimate mixture.   4 Process according to claim 3, characterized in that the carbon content of said mixture is   between 5.4 and 7.1 percent by weight.   Process according to Claim 4, characterized in that the aluminum oxide has a purity of 99.98% and   an average particle size of about 0.06 micro-   meter, and the carbon black has a purity of 97.6% and   an average particle size of about 0.027 micro-   Meter 6 Method according to Claim 4, characterized in that the chamber is heated to a temperature between 1550 and 1850 * C.   Process according to claim 5, characterized in that the chamber is first heated to a temperature of about 1550 ° C for about one hour, then at a temperature of 1750 ° C for about 40 minutes. Process according to Claim 5, characterized   in that the chamber is first heated to a temperature of   approximately 1620 ° C for about one hour, then at   a temperature of 1820 C for about 40 minutes.   Process according to Claim 6, characterized   in that the heating step consists of heating of   edge the chamber to a temperature located at the lower end of the temperature range to transform   gamma aluminum oxide unstable at the temperature   1 O is in the alpha-alumina mixture stable at temperature; and then heating the chamber to a temperature at the upper end of the temperature range to react the transformed mixture   of powder with nitrogen gas to produce oxynitrile aluminum tress.   Process according to Claim 2, characterized in that the reacted powder is crushed to produce   particles having a size of 0.5 to 5 micrometers.   The process according to claim 10, characterized   in that the powder is heated in the air until all the organic impurities present in the mixture are virtually eliminated.   12 Process for the preparation of aluminum oxynitride   homogeneous medium, characterized in that a mixture of aluminum oxide and carbon black is prepared, the carbon content of the mixture being in the range of 5.4 to 7.1 percent by weight; it is reacted in an atmosphere   of nitrogen in circulation said mixture-first at a time   at a temperature of about 1550 ° C. for one hour and then at a temperature of at least 175 ° C. for 40 minutes; the reaction mixture is crushed with grinding balls   alumina in methanol; and the powder is filtered   through a 0.037 mm mesh filter.   The method of claim 12, characterized   furthermore, in that the filtered powder is dried, and this powder is heated to air to eliminate all   organic impurities possibly present.   14 Process for manufacturing oxynitride bodies   sintered aluminum transparent, characterized in that a mixture of aluminum oxide and carbon black is prepared; said mixture is reacted in the presence of nitrogen and at a temperature between 1550 C and   1850 ° C; a compressed green body of shape   terminated from said mixture; the green body is placed in a sintering chamber; doping additives are introduced into the chamber, said additives comprising a   0 or more elements selected from yttrium and lantern   thane or compounds thereof; and sinter the body   green in an atmosphere of nitrogen at a temperature of   below 1900 'C but below the temperature of the lidus of aluminum oxynitride.   Process according to claim 14, characterized   in that the carbon content of said mixture is   between 5.4 and 7.1 percent by weight.   Process according to claim 14, characterized   in that the doping additives are in the vapor phase   during a part of the sintering step.   Process according to claim 14, characterized   in that, in the sintering step, the additives of   doping are transferred to that body and disseminate in that him one.   18. Process according to claim 17, characterized   in that the doping additives produce a phase   liquid at the edges of the fears during the grinding stage.   19. The process according to claim 15, characterized   in that the doping additives are mixed with said mixture.   Process according to claim 19, characterized   in that the doping additives do not represent   more than 0.5 percent by weight of said mixture.   The method of claim 14, wherein   in addition, the reacted mixture is divided into particles having a size of between 0.5 and 5 micrometers; and we   heats the reacted mixture to remove impurities organic teas.   22 Body made of cubic aluminum oxynitride,   characterized in that it has a density of at least 99% of the theoretical density, a direct transmission of at least   % in the wavelength range of 0.3 to 5 micro-   meters and a resolution angle of 1 mrad or less.   23 Doped aluminum oxynitride body,   it has a density of at least 99% of the theoretical density and a direct transmission of at least   % in the wavelength range of 0.3 to 5 micro- meters.   24 Aluminum oxynitride body according to the   23, characterized in that it is formed of oxy-   substantially single-phase aluminum nitride and is doped with boron and at least one member selected from yttrium and lanthanum.